Microstructure and lattice preferred orientation of calcite mylonites at the base of the southern Urals accretionary prism

Abstract

Emplacement of the southern Urals accretionary complex onto the East European Craton involved off-scraping of shallow marine limestones from the continental margin and their underplating to the base of the accretionary complex during the final stages of arc–continent collision. These limestones underwent localized ductile to semi-brittle deformation at upper-crustal levels, at temperatures close to 200 8C. Field observations, macroscopic fabric elements and crystallographic preferred orientations indicate a heterogeneous distribution of the deformation. Measurements from twinned calcite grains indicate that peak differential stress conditions during deformation attained c. 230 40 MPa. These differential stress conditions are further supported by the small size of both recrystallized and twinned calcite grains. Twinning and intracrystalline slip were the major contributors to the bulk deformation and lattice preferred orientation (LPO) formation. LPO evolved with progressive strain, from an initial constrictional fabric that is consistent with the macroscopic mineral lineation as the dominant macroscopic fabric element to a c-axis maximum normal to foliation as the rock underwent dynamic recrystallization and grain refinement during progressive simple shear.